Parent Company Only Financial Statements Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 51,008	$ 43,050	$ 23,090
Net Cash Provided by (Used in) Operating Activities [Abstract]
Share-based compensation expense	1,865	1,517	1,395
Depreciation of premises and equipment	8,142	7,950	7,826
Net Cash Provided by Operating Activities	95,166	102,476	77,214
Net Cash Provided by (Used in) Investing Activities [Abstract]
Cash assumed in business combination	339,372	0	17,177
Purchase of investment securities available-for-sale	(191,689)	(419,714)	(253,815)
Purchases of premises and equipment, net	(10,303)	(8,602)	(7,733)
Net Cash Provided by (Used in) Investing Activities	53,127	(300,084)	(51,608)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Cash dividends paid	(22,566)	(21,964)	(21,243)
Proceeds from directors' stock purchase plan and exercise of stock options	260	245	292
Restricted Stock, Value, Shares Issued Net of Tax Withholdings	(248)	0	0
Net Cash Provided by Financing Activities	125,902	43,383	149,850
Net Increase (Decrease) in Cash and Cash Equivalents	274,195	(154,225)	175,456
Cash and Cash Equivalents at Beginning of Year	381,940	536,165	360,709
Cash and Cash Equivalents at End of Year	656,135	381,940	536,165
Chemical Financial Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income	51,008	43,050	23,090
Net Cash Provided by (Used in) Operating Activities [Abstract]
Share-based compensation expense	1,865	1,517	1,395
Depreciation of premises and equipment	463	456	497
Equity in undistributed net income of subsidiary bank	(25,828)	(23,229)	(5,571)
Net increase in other assets	1,033	(975)	(1,140)
Net increase (decrease) in other liabilities	394	256	(290)
Net Cash Provided by Operating Activities	28,935	21,075	17,981
Net Cash Provided by (Used in) Investing Activities [Abstract]
Cash assumed in business combination	0	0	850
Purchase of investment securities available-for-sale	(4,755)	0	0
Purchases of premises and equipment, net	(147)	(365)	(174)
Net Cash Provided by (Used in) Investing Activities	(4,902)	(365)	676
Net Cash Provided by (Used in) Financing Activities [Abstract]
Cash dividends paid	(22,566)	(21,964)	(21,243)
Proceeds from directors' stock purchase plan and exercise of stock options	260	245	292
Restricted Stock, Value, Shares Issued Net of Tax Withholdings	(248)	0	0
Net Cash Provided by Financing Activities	(22,554)	(21,719)	(20,951)
Net Increase (Decrease) in Cash and Cash Equivalents	1,479	(1,009)	(2,294)
Cash and Cash Equivalents at Beginning of Year	4,481	5,490	7,784
Cash and Cash Equivalents at End of Year	$ 5,960	$ 4,481	$ 5,490